Well, I use a Roland SPD-S. It has some preloaded 808 sounds, but i'd rather find some samples on my computer and put them on the SPD-S, via the Compact Flash. The SPD-S is pretty expensive. If this is the only sound you want to use, you aren't gunna find a very cheap way. You could buy a cheap drum trigger module and a cheap external drum pad and use that, provided it has 808 sounds on it, or has some connectivity with a computer. You could use something connected straight to a computer, like the Synesthesia Mandala USB.

if i buy just a drum pad, like one for a roland kit, would i need some sort of drum module thing to run it through? actually im pretty sure i would but im just trying to figure out the cheapest option here...but could i just use a pad, ran through a module, hooked into the PA to do my 808 boom?